Multilevel Control Structures for Three Discrete Manufacturing Processes

Abstract Discrete manufacturing systems represent a large fraction of industrial processes and involve control problems whose dimension precludes straightforward solution. In general, such systems involve transformationfrom raw material to finished product and include a sequence of discrete operations (machining or parts, storage in and retrieval from inventory, etc.). Concornmitant with each of these operations are control problems including monitoring, reporting, scheduling, machine loading, start up, shut down, batch control, emergency actions, security enforcement, etc. The great differences among the disturbances and contingencies which can affect the operation of such processes suggests that the controls associated with each stage of the process and with each type or class of disturbance be handled as separate tasks. This decomposition of the control system, in turn, requires coordination between the subunits in order that the overall process objectives are achieved. Hierarchical control systems have proved to be effective models for the design of complex control systems for continuous processes. The objective of this paper is to demonstrate how a hierarchical structure can also lead to the effective control of discrete processes. Three examples are presented, each illustrating a different aspect of the discrete control problem formulation. It is shown, in each case, how the control tasks organize naturally into a multilayer hierarchical structure. The examples focus on problems of scheduling, control of discrete operations, and security control, the latter being control in the face of catastrophic disturbances. Generalizations are drawn from the experiences of these examples. Various features of the proposed structure are pointed up with specific reference made to problems of hardware and software implementation.